1. Field of the Invention
The present invention concerns in general magnetic resonance systems (or magnetic resonance tomography system) as used in medicine for examination of patients.
2. Description of the Prior Art
Magnetic resonance tomography is based on the physical phenomenon of nuclear magnetic resonance and is successfully used as an imaging method in medicine and biophysics. The water protons present in the body, or their nuclear spins, align in a strong, constant magnetic field. These nuclear spins can be brought out of the equilibrium state by the radiation of a radio-frequency pulse, and they relax again into this equilibrium state with specific relaxation times. These relaxation phenomena can be detected with suitable acquisition coils. The spatial resolution ensues by switching of additional currents in gradient coils for generation of magnetic field gradients. The precession frequency of the spins changes dependent on the location.
The acquisition of the data in magnetic resonance tomography ensues in k-space, or Fourier space. The magnetic resonance image (MR image) considered by the physician is obtained by a Fourier transformation of the MR raw data. Generally, gradients are activated in all three spatial directions for spatial coding of an examined subject. Differentiation is made between the slice-selection gradients, the phase coding gradients and the gradients during which the signal readout ensues.
In order to improve the signal-to-noise ratio, it is known to identically repeat a specific measurement sequence (i.e. a specific sequence of gradient switchings) and ultimately to average the signals of both measurements in order to present an averaged MR image that has a better signal-to-noise ratio. However, this leads to significantly longer acquisition times.